drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference

drm/fb-helper: Reduce READ_ONCE(master) to lockless_dereference

We are only documenting that the read is outside of the lock, and do not
require strict ordering on the operation. In this case the more relaxed
lockless_dereference() will suffice.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Julia Lawall <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Emil Velikov <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]